Processing, fine powders extrusion

In a nonaqueous copolymerization, fluorinated acyl peroxides are added that are soluble in the medium.43 A chain transfer agent may be added to control the molecular weight of the resin. The polymer is separated from the medium and converted into useful forms such as melt-extruded cubes for processes working with melt (e.g., extrusion, injection molding). The resins are also available as aqueous dispersions, molding powders, and fine powders for powder coating.44 45... 

Another application of PTFE dispersions is the preparation of a variety of compositions with other materials, such as mineral fillers, other polymers in powdered form by co-coagulation. The dispersion of the other component is blended with the PTFE dispersion and the blend is then coagulated. The resulting composition can be processed by extrusion with lubricants (see processing of fine powders) or by compression molding.16... 

A major requirement of paste extrusion is that, up to the point of sintering and coalescence, the ex-trudate must possess sufficient strength to withstand the extensive handling that takes place during the process. The tendency of fine powder to fibrillate (form a web of strong filaments between particles) when extruded provides the needed strength and the unique characteristics of fine powder articles. 

Northwestern University has developed a process they refer to as solid-state shear extrusion pulverization. In this process, a twin-screw extruder is used to convert mixed plastics and scrap rubber into a imi-form fine powder, which can then be used in a variety of products. 

The unique invention that led to PTFE polymer products expanding and becoming desirably porous made the material suitable for use as implants. The modified structure consists of nodes interconnected by very small fibrils (Fig. 1.11). Briefly, the process is as follows. A fine powder of un-sintered polytetrafluoroethylene is uniformly mixed with a Uquid lubricant and then shaped into a product (sheet, rod or tube) by extrusion. After removal of the... 

Fine Powder Resins. Fine powder PTFE resins are extremely sensitive to shear. They must be handled gently to avoid shear, which prevents processing. However, fine powder is suitable for the manufacture of tubing and wire insulation for which compression molding is not suitable. A paste-extrusion process may be applied to the fabrication of tubes with diameters from fractions of a millimeter to about a meter, walls from thicknesses of 100-400 /um, thin rods with up to 50-mm diameters, and cable sheathing. Calendering imsintered extruded solid rods produces thread-sealant tape and gaskets. 

Compression (Uniaxial) Molding. The mix in compression molding is usually a fine powder (flour) as opposed to the coarser material used in extrusion. Tungsten carbide dies are frequently used with pressures on the order of 28 to 280 MPa (4000 to 40,000 psi). Complex shapes can be produced by this process (Fig. 5.4). l i However, die-wall friction euid die edge effect may cause non-uniformity in the density and other properties of the finished product. 

Nylon 11 n (polyundecanamide) A type of nylon produced by polycondensation of the monomer 11-aminoundecanoic acid, a derivative of castor oil. It is available in the form of fine powders for rotational molding and other powder processes and in pellet form for extrusion or molding. Like nylon 12, nylon 11 has properties intermediate between those of nylon 6 and polyethylene good impact strength, hardness, and abrasion resistance, but other mechanical properties are lower than those of most other nylons. However, due to its exceptionally low water absorption, the dimensional stability of nylon 11 is high. A modified nylon 11 trade named Rilsan N is flexible, transparent, and self-extinguishing. 

In slurry and gas phase processes, the polypropylene leaving the reactor is in a fine powder form. This powder is then mixed with various chemical stabilizing and performance additives, fed to an extrusion compounding machine, which melts and mixes the materials and forces the melt through dies where the resin is cut into pellets, and cooled using water, then packaged for sale. 

Commercially PTFE is available in granular and fine powder resin forms and aqueous dispersions. The granular form of PTFE resin is prepared by suspension polymerization process in an aqueous mediiun with little or no dispersing agent. These forms of PTFE resins are mainly used for compression molding and ram extrusion. The fine PTFE powder is prepared by controlled emulsion polymerization and it is useful for paste extrusion into tapes, tubes, pipe liners, insulation layer of wires, gas-liquid separation membranes and fibers. 

The alloys are first produced by rapid solidification and are amorphous in nature. They are either directly fabricated as powders, by a process such as high-pressure gas atomisation (HPGA), or by melt-spinning of ribbons, which are subsequently pulverised to form a powder (<150 /im). The powders are then consolidated by hot extrusion between 950-1050°C where the initial amorphous structure breaks down and forms a fine dispersion of stable borides in a ductile Fe-based matrix. 

Ceramic particles in organic liquid media behave differently from those in aqueous media. Many known ceramic-processing techniques involve fine particles dispersed in nonaqueous media. Some of the better known examples are tape casting of barium titanate and aluminum nitride powders and tape or slip casting, extrusion, and injection molding of ceramic high Tc superconductor powders. 

Co-extrusion processing using thermoplastic binder systems for ceramic bodies has been described by Van Hoy [Hoy 98]. The major binder component used in this study was ethylene vinyl acetate, and methoxypoly-ethylene glycol was used as a plasticizer. Feedstocks of alumina, a piezoelectric ceramic powder and carbon black were repeatedly co-extruded to form an array of fine M -shaped structures. With this method the size of the alumina M shapes could be reduced by a factor of 915. In this example the use of solvent-based binders would have been problematical because of drying of the feedstocks and extrudates during the co-extrusion process and during handling. 

Plastic deformation of a moldable powder-additive mixture is employed in several forming methods for ceramics. Extrusion of a moist clay-water mixture is used extensively in the traditional ceramics sector for forming components with a regular cross section (e.g., solid and hollow cylinders, tiles, and bricks). The method is also used to form some oxide ceramics for advanced applications (e.g., catalyst supports, capacitor tubes and electrical insulators). A recent development is the repeated co-extrusion of a particle-filled thermoplastic polymer to produce textured microstructures or fine-scale structures. Injection molding of a ceramic-polymer mixture is a potentially useful method for the mass production of small ceramic articles with complex shapes. However, the method has not yet materialized into a significant forming process for ceramics mainly because of two factors ... 

A method based on repeated co-extrusion of a powder-filled thermoplastic polymer has been developed recently to form ceramics with a textured microstructure (59) or with fine-scale features (60). A schematic of the method is shown in Fig. 6.47. A mixture of ceramic particles (e.g., AI2O3), thermoplastic polymer (e.g., ethylene vinyl acetate), and processing aid (e.g., low molecular weight polyethylene glycol as a plasticizer) containing —50 vol% particles is formed into a rodlike feed material (the feed rod) with the required arrangement of the ceramic phase by extrusion in a piston extruder or by lamination. Extrusion of the feed rod. 

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